We have developed a platform strategy to treat the CNS manifestations of lysosomal storage disorders using adeno-associated virus (AAV)-mediated gene transfer. In a now completed NINDS grant (U01 NS047458), we have developed a 2nd generation AAV vector (AAVrh.10) that has markedly improved distribution of gene expression throughout the CNS. The AAVrh.10 vector has already been administered to the human CNS for late infantile neuronal ceroid lipofuscinosis (LINCL;a 1o grey matter disease). We now propose a milestone driven project to apply our experience with LINCL to metachromatic leukodystrophy (MLD), an autosomal recessive lysosomal storage disorder caused by a deficiency in arylsulfatase A (ARSA). Different from LINCL, ARSA deficiency is a grey and white matter disorder resulting from lysosomal accumulation of sphingolipid cerebroside 3-sulfate ("sulfatide") in the CNS resulting in a widespread demyelination, rapid neurologic decline, a vegetative stage and death a few years after diagnosis. In this project, we propose to develop sufficient preclinical efficacy data and toxicology data to obtain an allowed Investigational New Drug (IND) for the treatment of MLD by direct administration to the CNS of an AAVrh.l0-based vector coding for ARSA. In this revised UOI proposal, with extensive changes based on the recommendations of the reviewers, our overall goal therefore is to complete the preclinical efficacy, toxicology, manufacturing and regulatory requirements necessary to commence a human clinical study. We propose to achieve this goal with the following aims. Specific Aim 1. Utilizing a scalable manufacturing process, produce and qualify the AAVrh.l0 ARSA vector for the toxicology and clinical studies. Specific Aim 2. Assess toxicology, immune responses and ARSA distribution following AAVrh.l0-mediated ARSA administration to brain of rats and nonhuman primates. Specific Aim 3. Develop the clinical protocol and regulatory documents, and gain regulatory approvals for a clinical study.